Pneumatic pipelines convey particulate solids, as e.g. pulverized or granular materials, using a gaseous fluid, usually air or an inert gas, as carrier medium. As pneumatic pipelines are becoming widespread in many areas of industry, there is an urgent need for simple and reliable methods and devices for monitoring a flow of particulate solids in a pneumatic pipeline and, more particularly, for monitoring the mass flow rate thereof. Being capable of continuously monitoring the mass flow rate in pneumatic pipelines allows for example: (i) to warrant an accurate delivery of particulate solids, thereby allowing to optimize a process and/or achieve a better product quality; (ii) to adjust optimum conveying conditions, thereby optimizing energy consumption and wear in the pneumatic conveying system; and (iii) to balance flow rates between multiple pipelines in complex pneumatic conveying systems.
Over the past three decades many different methods and devices have been developed for continuously monitoring the mass flow rate in a two phase solid/gas flow, including: mechanical, electrostatic, microwave, optical and acoustic methods. None of these methods is however a satisfactory solution for monitoring a flow of particulate solids in a pneumatic pipeline.
Mechanical methods use mechanical flow meters, such as: impact plates, multi-bladed rotors, Coriolis wheels, pressure drop measurement devices, as e.g. orifice plates and Venturi tubes. However, such mechanical flowmeters are generally unsuitable for monitoring the flow rate in pneumatic pipelines, because they are too sensitive to abrasion and present a risk of pipeline blockage.
Electrostatic flowmeters sense the electrostatic charge carried by the moving particles in pneumatic pipelines. A variety of factors, such as physical and chemical properties, humidity and velocity of the solid/gas flow, as well as con figuration, material and wall roughness of the pipeline can influence their measures. Attempts have been made to improve accuracy by charging up the transported solid via an external electric source. However, this approach is considered unsuitable in many cases due to the risk of particulate explosion.
Microwave flowmeters generate electromagnetic waves in the GHz frequency range and measure how these electromagnetic waves are affected by the particulate solid flow in the pneumatic pipeline. A problem with the microwave flowmeters is that the electromagnetic field is generally not homogeneous over the whole cross section of the pipeline. The measurements may therefore be falsified by inhomogeneous flow regimes such as roping. Furthermore, microwave flowmeters are very sensitive to physical and chemical properties of the transported solids, and their measures are easily falsified by small depositions of solids.
Optical methods are base on light attenuation or scattering by the particulate solids flow. They are only applicable to gas/solids flows where solids concentration is very low and are therefore generally unsuitable for pneumatic pipelines.
Acoustic methods can be divided into active and passive acoustic methods. Active acoustic methods measure the attenuation of an incident ultrasonic beam by the solid particles conveyed in the pipeline. A problem with the active acoustic methods is that they do not provide a homogeneous coverage of the whole cross section of the pipeline and that their measurements are therefore falsified by inhomogeneous flow regimes such as roping. Passive acoustic methods monitor the structure-borne acoustic waves generated by moving particles impacting upon or sliding along the pipe walls. The structure-borne acoustic waves are detected by microphones or piezoelectric sensors, which are strapped to the outer surface of the pipeline. Best results are obtained if the acoustic sensors are mounted on the extrados surface of a bend in the pipeline. Usually a high frequency range is monitored to counteract the effects of mechanical noises conducted along the pipe. These passive acoustic methods are falsified by inhomogeneous flow regimes and by structure-borne acoustic noise generated along the pneumatic pipeline.
In “New design of the two-phase flow meters” published in 2000 in “SENSORS AND ACTUATORS, A” vol. 86, No3, pages 220–225, (publisher ELSEVIER SEQUOIA), P. BENES and K. ZEHNULA describe a method of measurement of small flow of two-phase media, where solid particles are carried by a carrying gas (air). The method is based on the principle that the solid particles carried by the flowing air generate an acoustic pressure wave when they impact onto a properly formed obstacle. The authors teach that the magnitude of the acoustic emission is proportional to the (average) mass of particles having a constant velocity. They suggest to use the method in two application areas: (1) mass flow or flow velocity measurement in different manufacturing areas, and (2) air dust measurement. In the first case, they use a rod that is inserted in the flow channel perpendicularly to the direction of flow and attach the sensor to this rod, so that the rod is used as a wave guide. It will be appreciated that this measuring method is not very reliable in case of inhomogeneous flow regimes in a pneumatic pipeline. In the second case (i.e. the air dust measurement), they concentrate the air flow directly on a piezoelectric sensor. Such a solution is of course only applicable to very low solid particles concentrations and certainly not to a particulate solids flow in a pneumatic pipeline.